Processes for producing flexographic printing plates from a photopolymerizable element are well known in the art; see e.g. Plambeck, U.S. Pat. No. 2,760,863; Suzuki et al., U.S. Pat. No. 3,556,791; Varga et al., U.S. Pat. No. 3,798,035; Kurka, U.S. Pat. No. 3,825,430; Recchia et al., U.S. Pat. No. 3,951,657. Typically, these processes include (1) exposing imagewise a photopolymerizable element to actinic radiation emitting a wavelength in the range of 365 nm, (2) removing the unexposed or unpolymerized areas of the plate with solvent and (3) drying the resulting plate. The photopolymerized element can then be detackified by exposing the element to ultraviolet radiation emitting a wavelength in the range of 254 nm. To ensure final plate hardening and photopolymerization, the printing element can be further post-exposed to radiation emitting at wavelengths in the range of 365 nm. Although processes such as these are commonly used, they have the disadvantage of producing plates that are dimensionally unstable over time.
Current platemaking processes utilize various sources of radiation for developing relief images and maximizing plate hardening. For example, actinic radiation from a variety of sources can be used, including commercial ultraviolet fluorescent tubes, medium, high, and low pressure mercury vapor lamps, argon glow lamps, photographic flood lamps, pulsed xenon lamps, carbon arc lamps, etc. It has been found that photopolymeric flexographic relief printing plates prepared using conventional radiation sources shrink and lose weight over time due to photoshrinkage, evaporation of volatiles and solvent extraction. Currently, photopolymer plate shrinkdown can be of such magnitude that, at times, the printing latitude of presses is exceeded and consequently, the gears bottom out and the printing run is aborted. Thus, reducing plate shrinkage is particularly important in the flexographic printing plate industry because maximum consistent print quality will be obtained using plates with reduced shrinkage.